Piezoelectric ceramics have a function whereby an electrical charge is generated on the surface in response to external stress (conversion from mechanical energy to electrical energy), and a function whereby an externally applied electric field generates strain (conversion from electrical energy to mechanical energy), and in recent years they have been widely applied in the fields of ultrasonic motors, sensors and the like.
The main composition used for such piezoelectric ceramics is a solid solution of PbZrO3 (PZ) and PbTiO3 (PT) (PZT system). The reason for their use is that excellent piezoelectric characteristics can be obtained by using compositions near the crystallographic phase boundary (Morphotropic phase boundary) between rhombohedral PZ and tetragonal PT. A wide range of PZT-based piezoelectric ceramics have been developed for a large variety of needs by adding various additives or materials, and properties suited for different fields of use are desired. Also, solid solutions composed mainly of lead-based perovskite compositions such as Pb(Mg,Nb)O3 (PMN) have been used as non-PZT-based piezoelectric ceramics.
However, these piezoelectric ceramics contain about 60-70 mass % lead oxide (PbO) as the major component. In PZT or PMN, for example, the mass ratio is about 2/3 lead oxide. This has led to concerns that large amounts of lead oxide may volatilize and diffuse into the air during production of such piezoelectric ceramics. Moreover, recovery of lead oxide in piezoelectric products is difficult, and elution of lead by acid rain when such products are exposed to outside air is another concern. From the viewpoint of protecting the environment, therefore, and considering the widening applications and increasing use of piezoelectric ceramics in the future, a demand exists for a piezoelectric ceramic with a satisfactorily reduced lead content.
The characteristics required for piezoelectric ceramics differ depending on the field in which they are used. For example, for high-power uses such as ultrasonic motors and piezoelectric transducers, it is desirable to have a high piezoelectric constant as preventing mechanical quality factor (Qm) reducing significantly during high-amplitude excitation. When used in an ultrasonic motor, such materials may be used under the high-temperature environment created by driving-induced self-heating, and therefore excellent piezoelectric characteristics must be exhibited in a wide temperature range.
Barium titanate (BaTiO3) and bismuth laminar ferroelectric substances are known as piezoelectric ceramics containing no lead as a constituent element. However, since barium titanate has a low Curie point of 130° C. and loses its piezoelectric characteristics at above that temperature, it cannot be used for purposes that require piezoelectric characteristics at high temperatures. On the other hand, bismuth laminar ferroelectric substances usually have Curie points of 400° C. and higher and exhibit excellent thermal stability, but reducing the lead content makes it difficult to achieve excellent piezoelectric characteristics. In addition, because of their high crystal anisotropy, they require orientation for spontaneous polarization by hot forging or the like, and therefore room remains for improvement in productivity.
As alternative compositions for piezoelectric ceramics there have been disclosed two-component solid solutions of bismuth sodium titanate ((Bi1/2Na1/2)TiO3) and bismuth potassium titanate ((Bi1/2K1/2)TiO3), and two-component solid solutions of bismuth sodium titanate ((Bi1/2Na1/2)TiO3) and bismuth lithium titanate ((Bi1/2Li1/2)TiO3) (for example, see Patent documents 1 to 3).    [Patent Document 1] Japanese Patent Application Laid-Open HEI No. 11-217262    [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-272962    [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-44335